Compressed air supply installations are used in vehicles to supply air suspension systems with compressed air. Air suspension systems may also comprise ride-height control devices, which serve for adjusting the distance between the vehicle axle and the vehicle body. An air suspension system of a pneumatic system comprises a number of air bellows, which are pneumatically connected to a common line (gallery) and which raise the vehicle body as they are increasingly filled with air and correspondingly lower it as the filling decreases. As the distance between the vehicle axle and the vehicle body or the ground clearance increases, the spring travels become longer and even greater unevenness in the ground surface can be overcome without coming into contact with the vehicle body. Such systems are increasingly being used for preference in all-terrain vehicles and sport utility vehicles (SUVs).
It is desirable with high-performance vehicles, particularly in the case of SUVs, to provide the vehicle on the one hand with a relatively low ground clearance for high road speeds and on the other with a relatively high ground clearance for off-road use. It is furthermore desirable to alter the ground clearance as rapidly as possible, which increases the demands on the responsiveness, flexibility and reliability of a compressed air supply installation.
In order to ensure the long-term working of the compressed air supply installation, a main pneumatic line of the compressed air supply installation comprises an air drier, which serves to dry the compressed air. This prevents moisture collecting in the pneumatic system. Moisture can lead at relatively low temperatures to the formation of valve-damaging crystals and moreover to unwanted defects in the compressed air supply installation and in the pneumatic unit. An air drier comprises a desiccant, usually a granulate filling, through which the compressed air can flow, so that the granulate filling is able to adsorb moisture contained in the compressed air. An air drier may possibly be designed as a regenerative air drier. This can be done by causing the dried compressed air from the pneumatic unit, in particular an air suspension system, to flow through the granulate filling in each venting cycle—generally in a countercurrent flow but also sometimes in a concurrent flow, relative to the filling direction. A regeneration of the air drier is afforded substantially through an alternating pressure on the air drier, in which a pressure prevailing during the regeneration is regularly lower than during adsorption, in order to allow the granulate to give off moisture. For this purpose the venting valve arrangement may be opened, the regeneration capacity of the air drier regularly varying as a function of the pressure ratios and the alternating pressure amplitude in the compressed air supply installation. For such a so-called alternating pressure adsorption, also, it has proved desirable to make a compressed air supply installation flexible and at the same time reliable. The aim, in particular, is to allow a relatively rapid venting on the one hand whilst nevertheless affording a low air pressure (i.e. during regeneration) alternating pressure amplitude sufficiently high for regeneration of the air drier.
Various attempts have been made to design a pneumatic connection between an aforementioned compressed air feed and an aforementioned pneumatic unit. These take account of the basic functions of a compressed air supply installation in ventilating the pneumatic unit and venting the pneumatic unit. These still allow for improvement, however, with regard to the aforementioned need for a relatively rapid venting in conjunction with a low and/or maximum air pressure or alternating pressure amplitude which is nevertheless sufficient for regeneration of the air drier.
DE 35 42 974 A1 discloses a ride-height control device for vehicles, provided with air filters, which serves for setting a predefined distance of the vehicle cell from the vehicle axle as a function of the vehicle loading by filling or emptying the air springs. The device comprises a safety valve that can be controlled by the pressure in the air springs. In such an installation a regeneration of the air drier is possible by way of a restrictor and a non-return valve opening in the opposite direction to a filling direction. The compressed air supply installation in DE 35 42 974 A1 has long proved successful but is still capable of improvement. It has been shown that although the installation is advantageously suited to saving compressed air also in a regeneration of the air drier, by using the first non-return valve to separate the compressed air supply installation and the pneumatic system, the applicant's installation disclosed in DE 35 42 974 A1 has nevertheless proved limited in its scope, particularly for advanced applications which call for relatively flexible and rapid compressed air handling in a comparatively short space of time.
DE 10 2010 054 704 A1 describes a compressed air supply installation for the operation of a pneumatic unit, having an air feed and an air compressor for supplying a compressed air feed with compressed air. A venting line has a venting valve arrangement and a venting connection for discharging air and a compressed air supply line has an air drier and a compressed air connection for supplying the pneumatic unit with compressed air, wherein the air drier comprises a drying vessel containing a desiccant through which compressed air can flow. The compressed air supply installation provides for a solenoid valve arrangement as a directly controlled venting solenoid valve arrangement which, activated via a control line, allows direct control of the overall compressed air volume. The single valve of the solenoid valve arrangement allows a rapid and flexible venting of the pneumatic unit and the compressed air supply installation without the need for an additional control valve. This primarily affords a compact compressed air supply installation, but as an advantageous basis for improved acoustics and/or regeneration of an air drier this concept is still particularly open to improvement.
DE 10 2011 109 500 A1 discloses an inherently particularly advantageous compressed air supply installation, in which an isolating valve in a main pneumatic line is formed as a pneumatically releasable non-return valve. In a development a double relay piston may be provided, which can be subjected to pressure in such a way that the non-return valve can be released whilst at the same time actuating a venting valve.